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What frame material should I choose? This should be one of the key decisions many people make when buying a fat tire electric bike.
Over the years, ebicycles frames have been made from a variety of different materials, initially dominated by steel. The main choices now are aluminum and carbon fiber, although steel and titanium still have many users in the market. Each frame material has its pros and cons, depending on personal needs such as weight, budget, longevity, and the performance characteristics you want from your frame.
Next we’ll outline some key properties of materials like aluminum, steel, titanium and carbon fiber to consider before buying a bike.
It’s common to see aluminum frames referred to as “alloys.” This is because pure aluminum is too soft to be shaped into a form for bicycle frames, so it is mixed with other elements to change its physical properties. In fact, all metal bike frames are made of alloy for the same reason. Steel itself is an alloy of iron, while titanium is primarily an alloy with aluminum and vanadium.
Numbers such as 6061 and 7005 (the two most commonly used aluminum alloys) are codes for the aluminum mixed with the additives each alloy forms (mainly silicon and magnesium). Each "recipe" will have slightly different properties. Aluminum alloy frames are also the metal of choice for mid-range budget frames, offering an unrivaled combination of light weight, high rigidity and economy.
How to make an Aluminum alloy frame?
All metals have very similar strength-to-weight ratios, and factors like the width and wall thickness of the tubes are more important than their strength. It is relatively easy to manipulate aluminum tubes to have different properties along their length, and aluminum tubes are often butted in a way that ensures the stiffness where it is needed and saves weight where it is not.
The pipes of aluminum frames are usually welded. This means it's thicker at the ends, where the stress is greater and requires more material to join the other tubes, and thinner in the middle to save weight. Straight rail tubes are consistently thicker and provide consistent performance. Single butted tubes are thicker on one end (for example, at the bottom bracket connection), double butted tubes are thicker on both ends, and multiple butted tubes further reduce thickness in the middle of the tube.
Docking is only one part of production, we also frequently hear about the hydroforming process. It is the process of adjusting the shape of a tube by using high-pressure fluid to form the tube to fit a mold.
High-end aluminum frames are more advanced than ever, and the process can help form complex shapes to affect the characteristics of specific parts of the frame, including weight, strength, and comfort. Many of the latest aluminum frames also feature aerodynamic design features.
Aluminum tubes are often welded together to make the frame. The stock welds can be very bumpy in appearance, but this is usually sanded down after welding to give a flatter look, and it also helps save a bit of weight. After the aluminum frame is welded, it is often heat treated to restore the alloy to its full strength. Because a large amount of heat is applied to a local area when welding, this will change the properties of the local metal, making the weld the weakest area. Tempering after welding can help alleviate this situation.
Some early alloy frames were bonded, with tubes cut to length glued into lugs. Italian brand Alan and French brand Vitus were early adopters of this manufacturing method.
What are the characteristics of aluminum alloy frames?
While aluminum is the material used in most entry-level bike frames, it still has some pricier, performance-focused frame options, both on road and mountain bikes. Aluminum alloy frames are strong and durable, about one-fifth the price of carbon fiber, and aluminum is much less dense than steel.
Aluminum frames aren't exactly known for comfort due to the material's relative rigidity and lack of shock absorption, but the latest frame-building techniques, along with the trend towards wider tires on road bikes, mean many aluminum-framed bikes have improved ride quality. While aluminum frames offer a balance of strength, rigidity, and weight, unlike steel and titanium, aluminum alloys are prone to metal fatigue.
However, compared to carbon frames, aluminum frames are less likely to break upon impact. As a result, aluminum remains a popular frame material for mountain bikes and an entry-level road bike option on a budget.
Advantages of aluminum frames
- Good rigidity to weight ratio
- Load capacity is excellent
- More durable than carbon fiber frame
- Disadvantages of aluminum frames
- Average comfort
- Will produce metal fatigue
- difficult to repair
For about 100 years, steel was the primary material for frame construction until the mid-1990s, when aluminum frames began to dominate. However, there is still a market for steel bikes, and it is the primary material of choice for enthusiasts and custom builders.
Casing or welding
There are two main methods of manufacturing steel frames, sleeved and uncased. Casing frames insert tubing into cast steel lugs at the joints and then braze them together. Uncased frames are pipe joints welded together or fillet welded. Bushing construction used to be common, with classic steel frames often having very fine lugs, but welding tends to make the frame lighter and requires less cleanup than brazing, which often results in joints with excess material needs to be removed later.
Brazing, on the other hand, requires lower temperatures than welding, so the properties of the steel may change less, resulting in a stronger joint. Like the tubes in the aluminum alloy frame, the steel frame is usually butted to reduce weight. The tube shapes of the new steel frame are more diverse than the circular cross-section of the old model.
Wide range of steel materials
Most performance-focused steel bikes are made from chromium steel, with high-strength steel found on cheaper, lower-end frames. Steels of different compositions have different properties. Reynolds 531 is a classic alloy used for bicycle frames, where 531 refers to the ratio of manganese, carbon, and molybdenum in the alloy.
However, there are now more options, with tube set suppliers including Dedacciai and Columbus. Steel tubing comes in a wide variety of compositions and profiles, unlike other frame tubing materials. This means it’s possible to create a very strong touring frame or a lighter one, depending on the user’s needs.
Stainless steels are also an option, but they are weaker than chromium steels, although bicycle manufacturing has developed specialty stainless steels that are as strong or stronger than chromium steels.
Steel frame features
The main disadvantages of steel are weight and cost. It's heavier than aluminum and more expensive to manufacture on a large scale, so aluminum is still the preferred choice for most metal bike frames today.
While steel is much denser and heavier than aluminum, it is also stronger and more durable. This means that frame builders can use smaller diameter, thinner walled tubes and maintain the required level of stiffness, but the weight penalty is still there. Unlike aluminum, it can be repaired relatively easily if damaged. Likewise, steel has a fatigue limit and can withstand an infinite number of stresses below the fatigue limit without causing failure, unlike aluminum, which wears out over time.
Steel's natural properties also allow for “springy” comfort, so steel is a popular choice for boutique and custom bikes, as well as touring bikes, where weight is less of an issue. Steel offers a higher value than titanium, durability, and longevity are key. That is, unless made of stainless steel, the steel frame will need to be painted to stop corrosion on the outside, and the tubes can also be painted on the inside to prevent rust.
Advantages of steel frame
- Sturdy and durable without fatigue
- Easy to repair
- Comfort is relatively high
- More affordable than titanium
- Disadvantages of steel frames
- More expensive than aluminum
- The tube shape is not as advanced as aluminum
- Will corrode if not handled properly
Titanium is often viewed as a high-end option, in part due to its ride quality and cost, as well as the fact that titanium bikes are often touted as "bikes for life."
Most metals have a definite number of load cycles before they may fail. Titanium is much more resilient to repeated stress and strain, meaning skilled frame builders can create lighter, more compliant frames without the risk of failure.
Like aluminum and steel, titanium is an alloy, and frame manufacturers have different grades to choose from. AL3 2.5V (containing 3% aluminum and 2.5% vanadium) is the most commonly used grade for titanium frames. 6AL 4V tubes are significantly stronger and therefore harder to exploit, and are sometimes used on high-performance frames or in individual areas where stiffness is critical, such as the head tube and bottom bracket.
Titanium is also highly fatigue-resistant, meaning it absorbs shock better and won't deform. For example, Moots uses the flexibility of titanium to create shock-absorbing systems in the seatstays of its Mountaineer and Routt YBB framesets.
Titanium alloy frame characteristics
Compared to other metals, titanium has clear advantages when it comes to bicycle frames. It's less dense than steel, so the frameset can be lighter while still having thicker-walled tubes. Titanium tubes weigh half as much as steel tubes of the same tensile strength. Enigma's titanium tube stacks are typically 0.9 mm thick at their thinnest point, while steel tube stacks are 0.5 mm thick.
This makes the titanium frame harder to dent, and since titanium doesn't corrode, the frame doesn't need to be painted. However, titanium is more difficult to process than steel, and attention must be paid to the cleaning and control of the welding process, and argon gas protection is required to prevent oxidation.
In the past, there were very few titanium tube sets available, and they weren't designed specifically for bikes. This isn't an issue these days, as there are more options for bike-specific tubes, and design features like tapered head tubes and wider bottom bracket standards mean titanium frames can be designed to be as strong as the application demands.
Advantages of titanium frame
- Sturdy and durable without fatigue
- lighter than steel
- Good ride quality
- No special painting required
- Disadvantages of titanium frame
- Difficult to process
- Not as light as carbon
- relatively expensive
Ever since Lance Armstrong won the 1999 Tour de France with the Trek 5500 OCLV, carbon fiber has been the material of choice for performance bike frames.
There's a good reason for this. Carbon fiber is a highly adaptable material that can be shaped and fine-tuned to precise requirements, balancing stiffness, comfort and aerodynamics. Carbon is not without its drawbacks, though. Carbon frames are expensive, running into five figures at the high end of the market, and are more susceptible to impact damage than other materials.
How is a carbon fiber frame made?
Carbon fiber frames consist of layers of carbon fiber (fiber braid) embedded in an epoxy resin matrix. The carbon fiber gives it strength and the resin holds it together.
Carbon has the highest strength-to-weight ratio per layer, but that's only in one direction, so it's stacked on the bike frame at multiple angles. This means its strength-to-weight ratio is slightly down, but still higher than other materials.
Carbon fiber is also brittle, so it is very easy to be damaged in areas such as the tail hook. This is why carbon fiber bikes often come with metal thru axle stems for mounting the thru axles, or have metal protective blocks built into these areas.
An impact can also cause damage to the interior of the frame, which may not be visible from the outside and is likely to cause the frame to break unexpectedly. If a carbon fiber frame has been hit or severely damaged, it should be inspected before riding again. Ultrasound or X-rays may be used to check for damage inside the frame.
If a carbon fiber frame does become damaged, recycling options are very limited, whereas metal frames are easily recycled. The value of titanium in particular means it's less likely to end up in a landfill.
Advantages of carbon frame
- Excellent rigidity-to-weight ratio
- Strong plasticity
- Disadvantages of carbon frame
- Easily damaged after collision